1. Field of the Invention
The present invention relates to a surface shape measurement method that divides a surface shape of an optical element so as to measure the surface shape.
2. Description of the Related Art
As a method of measuring a surface shape of an object surface (an optical element), particularly an aspherical lens, without contacting the surface at high speed, Jahannes Pfund, Norbert Lindlein and Johannes Schwider, “NonNull testing of rotationally symmetric aspheres: a systematic error assessment,” App. Opt. 40 (2001) p. 439 discloses a method of illuminating light of a spherical wave onto an object surface via an optical system to measure reflected light of the object surface using the Shack-Hartmann sensor of a light receiving unit. In a method of disclosing in Japanese Patent Laid-Open No. 2006-30016, in accordance with a diameter of the object or an aspherical surface amount (a deviation from the spherical surface), a wavefront that exceeds a size of the diameter of the Shack-Hartmann sensor needs to be measured. In this case, in view of reducing the size and the cost of a measurement apparatus, a method of dividing a sensor incident wavefront into partial regions to perform a division measurement and then performing a data processing to stitch each of division measurement data (partial region data) is suitably used (a stitching method). In the data processing of the stitching method, correcting an error generated by the division measurement is important.
In Japanese Patent Laid-Open No. 2006-30016 and J. Floriot, X. Levecq, S. Bucourt, M. Thomasset, F. Polack, M. Idir, P. Mercère, S. Brochet, and T. Moreno, “Surface metrology with a stitching Shack-Hartmann profilometric head,” Proc. of SPIE Vol. 6616, 66162A(2007), measurement apparatuses that perform a measurement using the stitching method using the Shack-Hartmann sensor are disclosed. In Japanese Patent Laid-Open No. 2006-30016, a method of removing an error caused by an inclination of a sensor of errors contained in each the division measurement data is disclosed. In J. Floriot, X. Levecq, S. Bucourt, M. Thomasset, F. Polack, M. Idir, P. Mercère, S. Brochet, and T. Moreno, “Surface metrology with a stitching Shack-Hartmann profilometric head,” Proc. of SPIE Vol. 6616, 66162A(2007), a method of removing an error caused by a defocus component in addition to the inclination error of the sensor is disclosed.
However, in the methods disclosed in Japanese Patent Laid-Open No. 2006-30016 and J. Floriot, X. Levecq, S. Bucourt, M. Thomasset, F. Polack, M. Idir, P. Mercère, S. Brochet, and T. Moreno, “Surface metrology with a stitching Shack-Hartmann profilometric head,” Proc. of SPIE Vol. 6616, 66162A (2007), position displacements in an optical axis plane of the sensor and in an optical axis direction and an error caused by a rotation displacement around the optical axis (an error generated by a relative movement between an object and a sensor) cannot be corrected. In addition, a correction accuracy of the error generated by the relative movement between the object and the sensor is deteriorated.